o-Nitrophenol is more volatile than p-nitrophenol. It is due to

o-Nitrophenol involves intramolecular whereas p-nitrophenol involves intramolecular hydrogen bondings. O-Nitrophenol forms intramolecular H bond whereas molecules of P-Nitrophenol get associated through intermolecular H bond. During boiling, the strong intermolecular H bonding increases the boiling point but intramolecular H bonding cannot do so. Therefore, O-Nitrophenol is more steam volatile than P-Nitrophenol.